Cloudy pool water almost always comes from one of three things: not enough chlorine, pH or alkalinity too high, or a filter that is not keeping up. Test the water first. Then raise chlorine to shock level if it is low, bring pH to 7.2–7.8, and run the filter around the clock and clean it. Most pools clear in one to three days.
The chlorine that is working right now — it kills germs and algae.
pH
How acidic the water is. Right, chlorine works and eyes do not sting; wrong, it damages the pool.
Alkalinity
ppm
Total alkalinity holds pH steady, so it does not swing up and down.
Calcium
ppm
Calcium hardness. Too little eats plaster and heaters; too much leaves white scale.
Stabilizer
ppm
Cyanuric acid (CYA). It shields chlorine from the sun, so outdoor pools need some.
Type the size of your pool and at least one test result.
Step by step
1
Test free chlorine, pH, alkalinity and stabilizer (CYA) — a test strip or a drop kit is fine.
2
Type the numbers into the calculator on this page: it lists what to add, in a safe order, in amounts for your pool.
3
If free chlorine is low, shock the pool: raise chlorine to the shock level your stabilizer needs (40% of CYA, and at least 10 ppm).
4
Bring pH to 7.2–7.8 and alkalinity to 60–100 ppm (60–80 in a salt pool).
5
Run the pump 24 hours a day until the water clears. Brush the walls and floor, and clean or backwash the filter when its pressure rises.
6
Still cloudy after three days of good chlorine? A clarifier helps the filter catch fine particles; a floc drops them to the floor to vacuum out.
The product label always wins — if it says something different, follow the label.
Why it happens
Cloudy water is millions of tiny particles the filter is not catching: dead algae, dust, pollen, sunscreen — or calcium coming out of the water when pH and alkalinity run high.
Low chlorine lets algae start growing before you can see any green, and the first sign is a dull, hazy look.
Questions
Why is my pool cloudy after shocking?
Shock kills algae and bacteria, and the dead particles float until the filter catches them. Keep the pump running and clean the filter; it usually clears in a day or two.
Can I swim in a cloudy pool?
Not if you cannot see the floor of the deep end: nobody could see a swimmer in trouble. Wait until it clears.
My chlorine is fine but the pool is still cloudy. Why?
Check pH, alkalinity and calcium hardness: when they run high, calcium comes out of the water as a haze. Then check the filter — dirty, clogged or too small for the pool.
Clarifier or flocculant?
Clarifier clumps fine particles so the filter can catch them: easy, and slower. Floc drops them to the floor for you to vacuum to waste: faster, but it needs a filter valve with a Waste setting.